Sewage disposal of coastal cities submarine outfalls

REGIONAL SITUATION

Common practice for coastal cities is to discharge untreated wastewater to the nearest or most convenient water body and usually minimum considerations are given to the ensuing environmental and health consequences, primarily due to the lack of economic resources. In the majority of the coastal cities of Latin America and the Caribbean, raw sewage discharges occur on or very near bathing beaches. Geometric average levels of total coliforms in excess of 100 000 MNP/100 ml have frequently been observed in public bathing beaches with individual measurements at times approaching levels of raw sewage. The problems associated with near shore discharge of untreated sewage are aesthetic, can cause potential health and ecological hazards and often bring economic consequences due to curtailed tourism.

Based on surveys conducted by CEPIS and updated in 2006, there are 138 existing or planned submarine outfalls of 500 meters or greater in Latin America and the Caribbean:

The minimum outfall length of 500 meters was utilized as a reference point for the above table, although submarine outfalls longer than 500 meters would usually be required for major sewage discharges to comply with bathing beach coliform standards.

It is noted that only 30 of these outfalls serve cities of populations greater than 100 000 and in most cases these cities are only partially served. In 1995 there were 115 cities with populations greater than 100 000 in Latin America and the Caribbean. As such, the currently served population is comparatively small.

The economic situation of Latin America may not permit the adaptation of control measures utilized in developed countries. For example, secondary treatment applied in developed countries may not be a viable alternative in terms of costs for the countries of the Region and perhaps not necessary to achieve water quality goals. On the other hand, submarine outfalls provide an efficient, safe and relatively economic technology for the final disposal of sewage which, when designed appropriately and combined with preliminary or primary treatment, can achieve water quality objectives and minimize adverse environmental, ecological and public health impacts. For the year 2000, the population that lives on the coast or near estuaries and that could be potentially served by submarine outfalls, is estimated to be 110 million with an approximate wastewater flow of 250 m3/s. The appropriate disposal of this wastewater is critical to the future sustainable development and the environmental well-being of the Region.

The Pan American Center for Sanitary Engineering and Environmental Sciences (CEPIS, Spanish acronym), specialized center of the Division of Health and Environment (HEP) of the Pan American Health Organization/World Health Organization (PAHO/WHO), has developed general guidelines for the conceptual design of submarine outfalls and other related subjects taking into account various technologies presented in manuals of other international organizations, such as the World Bank and the Monitoring and Assessment Research Centre (MARC).

TThe intention of CEPIS is to provide basic concepts and requirements for the design of submarine outfalls and develop the capacity of the national water and sewage institutions to establish terms of reference for such projects and adequately review the quality of the work during the proposal and execution stages.

DOCUMENTS

• SALAS, H. (2000). Submarine outfalls A viable alternative for sewage discharge of coastal cities in Latin America and the Caribbean. Lima, CEPIS. (English and Spanish).

• SALAS, H. (2000). History and application of microbiological water quality standards in the marine environment. Lima, CEPIS. (English and Spanish).

• SALAS, H. (2000). Submarine outfalls General overview, basic design concepts and data requirement for Latin America and the Caribbean. Lima, CEPIS. (English and Spanish).

• REIFF, F. (2002). Small diameter HDPE (High-density polyethylene) submarine outfalls. Lima, CEPIS. (English).
  The document is being updated and soon to be published as a chapter in a book, "Design of Marine Wastewater Outfalls and Treatment Systems".
  

• GARCIA AGUDO, E. & LUDWIG, R. (1992). Milli-screens: pre-treatment system. Edited by H. Salas. Lima, CEPIS, 2000. (English).

• LUDWIG, R. (1988). Environmental impact assessment. Siting and design of submarine outfalls. MARC report number 43.
  Monitoring and Assessment Research Centre & World Health Organization. London. (English version). Translated by H. Salas, Lima, CEPIS (Spanish version).

The above publications are available at the BVSDE web site